Uncertainty in Measured Raindrop Size Distributions from Four Types of Collocated Instruments

• Main Authors: Wei-Yu Chang, GyuWon Lee, Ben Jong-Dao Jou, Wen-Chau Lee, Pay-Liam Lin, Cheng-Ku Yu
• Format: Article
• Language: English
• Published: MDPI AG 2020-04-01
• Series: Remote Sensing
• Subjects: drop size distribution; uncertainty; sampling volume; disdrometer; measuring principle
• Online Access: https://www.mdpi.com/2072-4292/12/7/1167

Four types (2D-video disdrometer: 2DVD; precipitation occurrence sensor system: POSS; micro-rain radar: MRR; and Joss–Waldvogel disdrometer: JWD) of sixteen instruments were collocated within a square area of 400 m² from 16 April to 8 May 2008 for intercomparison of drop size distribution (DSD) of rain. This unique dataset was used to study the inherent measurement uncertainty due to the diversity of the measuring principles and sampling sizes of the four types of instruments. The DSD intercomparison shows generally good agreement among them, except that the POSS and MRR had higher concentrations of small raindrops (<1.0 mm) and offered a better chance to observe big raindrops (>5.2 mm). The measurement uncertainty (<inline-formula> <math display="inline"> <semantics> <mi mathvariant="sans-serif">σ</mi> </semantics> </math> </inline-formula>) was obtained quantitatively after considering the zero or non-zero measurement error covariance between two instruments of the same type. The results indicate the measurement uncertainties were found to be neither independent nor identical among the same type of instruments. The MRR is relatively accurate (lower <inline-formula> <math display="inline"> <semantics> <mi mathvariant="sans-serif">σ</mi> </semantics> </math> </inline-formula>) due to large sampling volumes and accurate measurement of the Doppler power spectrum. The JWD is the least accurate due to the small sampling volumes. The <inline-formula> <math display="inline"> <semantics> <mi mathvariant="sans-serif">σ</mi> </semantics> </math> </inline-formula> decreases rapidly with increasing time-averaging window. The 2DVD shows the best accuracy of R in longer averaging time, but this is not true for Z due to the small sampling volume. The MRR outperformed other instruments for Z for entire averaging time due to its measuring principle.